patch 9.1.0437: Motif requires non-const char pointer for XPM  data

Problem:  Motif requires non-const char pointer for XPM data shared with
          GTK (Tony Mechelynck, after v9.1.0432)
Solution: Cast non-const to const char pointer for GTK (Drew Vogel).

closes: #14834

Signed-off-by: Drew Vogel <dvogel@github>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/vim32x32.xpm b/runtime/vim32x32.xpm
index 017b93b..43a3a40 100644
--- a/runtime/vim32x32.xpm
+++ b/runtime/vim32x32.xpm
@@ -1,5 +1,5 @@
 /* XPM */
-static const char * vim32x32[] = {
+static char * vim32x32[] = {
 "32 32 8 1",
 " 	c None",
 ".	c #000000",